OBJECTIVE To identify the social, behavioral, and hormonal mechanisms underlying the onset of polygyny in families of marmosets. RESULTS Female marmosets living with the natal family typically undergo reproductive suppression, failing to ovulate in the presence of the mother. The occurrence of such suppression is variable, however, as females have occasionally been reported to ovulate while housed with the natal group, and as both captive and wild groups have sometimes been reported to contain two breeding females. In this study, we are testing the hypothesis that females will escape from reproductive suppression in response to the introduction into the family of an appropriate mate, i.e., an unrelated adult male. Behavioral and hormonal data are collected from post-pubertal females and their mothers both while the family is intact and after replacement of the breeding male by an unrelated, unfamiliar adult male. Preliminary results indicate that post-pubertal, anovulatory females can be stimulated to ovulate in 1-3 weeks following introduction of an unrelated male. Moreover, these females may engage in sexual interactions with the novel male and eventually conceive. These findings indicate that reproductive failure in subordinate female marmosets may be mediated not only by rank-related suppression but also by the lack of appropriate, unrelated mates in the natal group. Moreover, they suggest that differences between marmoset groups in the number of breeding females may be attributable to differences in the relatedness of group members. FUTURE DIRECTIONS We will continue to investigate the neuroendocrine and sensory mechanisms by which introduction of an unrelated male leads to the onset of reproductive function in marmoset daughters, and the outcome of daughtersU reproductive attempts in these families. KEY WORDS mating systems, monogamy, fertility, social relationships, reproductive suppression, sexual behavior. FUNDING NSF IBN-9604321